1. Field of the Invention
This invention relates to apparatus and methods for reducing the viscosity of crude oil produced from a subterranean formation in order to facilitate pumping and/or transporting the oil.
2. Description of the Prior Art
The production of crude oil from an oil reservoir is generally assisted to a great extent by naturally occurring forces associated with the reservoir. These naturally occurring forces include the expanding force of natural gas, the buoyant force of approaching water and the force of gravity. Primary recovery techniques utilize these forces to cause the oil to migrate from the formation into the well bore. Unfortunately, the natural forces are typically only sufficient to allow a small percentage of the total oil in the reservoir to be produced.
Secondary recovery techniques are generally employed to recover more of the oil in the formation. These techniques utilize extraneous energy forces to supplement the naturally occurring forces in the formation and force the oil from the formation into the well bore. The extraneous forces can be generated from a large variety of sources including gas injection, steam injection and water injection. Secondary recovery techniques are typically initiated even before the primary forces of the reservoir are exhausted.
Water flooding is one example of a secondary recovery technique that has been successfully employed in different types of formations. Generally, in accordance with water flooding techniques, one or more injection wells and one or more production wells are utilized. An aqueous solution is injected through the injection well(s) in order to drive the oil to the production well(s) where it can be produced. Many modifications to basic water flooding techniques have been developed. These modifications include the use of certain chemicals and materials in the injection water to help displace the oil from the formation. For example, thickening agents are often employed to thicken the water and thereby increase its efficiency in driving the oil to the producing well(s). Surfactants have been employed to reduce the surface tension of the oil in the formation and thereby facilitate its production.
Aqueous alkaline solutions, e.g., caustic solutions, have been successfully utilized for flooding certain types of reservoirs. For example, alkali metal hydroxides such as sodium hydroxide react with organic acids present in the oil and depress the interfacial tension between the oil and the water resulting in emulsification of the oil. The emulsified oil is more easily displaced from the formation. This type of secondary recovery technique is often referred to as caustic flooding.
Another secondary recovery technique that has been employed to increase the recovery of oil in certain situations involves the use of sonic energy. For example, sonic stimulation has been utilized in Russia to improve oil production in depleted water flooded and water-dry oil reservoirs. The sound waves generally function to heat and reduce the viscosity of the oil, increase the permeability of the formation and generally induce migration of the oil to the well bore.
Secondary recovery techniques involving heavy and highly viscous crude oil ("heavy crude oil") are especially challenging. In order to efficiently produce heavy crude oil, the viscosity of the oil must be substantially reduced. Transportation of heavy crude oil (e.g., by pipeline) can also be difficult to accomplish in an efficient manner unless the viscosity of the oil is first reduced. Numerous techniques have been employed to reduce the viscosity of heavy oil. For example, U.S. Pat. No. 3,823,776 to Holmes discloses a process for increasing the recovery of heavy oil having a low acid value whereby an oxygen-containing gas is injected into the formation to oxidize the oil and establish an in situ combustion zone in the formation. An aqueous caustic solution is then injected into the well to quench the in situ combustion zone and react with organic acids present in the oil to facilitate production of the oil. U.S. Pat. No. 2,670,801 to Sherborne discloses that ultrasonic energy (10 to 3,000 kHz) facilitates recovery of heavy oil by in situ heating of the oil droplets and emulsification of the droplets to a water phase saturated with gas.
Unfortunately, the techniques utilized heretofore to facilitate recovery of heavy oil from subterranean formations are often not very successful. The cost of reducing the viscosity of heavy oil to a level whereby the oil can be lifted out of the formation and transported for further processing often exceeds the potential gain to be realized by producing the oil. Accordingly, there is a need for an improved apparatus and corresponding process for treating heavy crude oil produced from a petroleum reservoir whereby the viscosity of the oil can be substantially reduced and the oil can be produced and transported for further processing in an economical and efficient manner.